It is generally known that process plants, such as refineries, chemical plants or pulp and paper plants, consist of numerous process control loops connected together to produce various consumer products. Each of these process control loops is designed to keep some important process variable such as pressure, flow, level, or temperature, within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates load disturbances that affect the process variable and control of the process control loops within the plant. To reduce the effect of these load disturbances, the process variables are detected by sensors or transmitters and communicated to a process controller. The process controller processes this information and provides changes or modifications to the process loop to get the process variable back to where it should be after the load disturbance occurs. The modifications typically occur by changing flow through some type of final control element such as a control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or a chemical compound, to compensate for the load disturbance and maintain the regulated process variable as close as possible to the desired control or set point.
It is generally understood that various control valve configurations may be specifically applicable for certain applications. For example, when a quick-opening valve with a narrow control range is suitable, a rotary control valve, such as a butterfly valve, may be used. Alternatively, when precise control over a large control range is required, a sliding stem control valve may be used. In any configuration, such control valves are generally coupled to a control device such as an actuator, which controls the exact opening amount of the control valve in response to a control signal. Thus, when designing a process, the process engineer must consider many design requirements and design constraints. For example, the design engineer must determine the style of valve used, the size of the valve, the type of actuator, etc.
In some systems, especially in pneumatically controlled fluid process systems, the actuator for any given fluid process control device may include a diaphragm actuator. Typical diaphragm actuators comprise a housing containing a spring-biased diaphragm assembly. The diaphragm assembly is operatively coupled via a stem, or other actuator rod, to control the opening amount of the fluid process control device.
One known diaphragm assembly comprises a diaphragm and one or more diaphragm plates. The diaphragm comprises a flexible disk-shaped member constructed of a fluid-tight fabric, polymer, or other suitable material. The plates are disposed adjacent to the diaphragm and are adapted to be engaged by one or more springs disposed within the housing. Additionally, the plates provide a rigid mechanical connection to the stem. The springs serve to bias the diaphragm assembly into a predetermined position such that the actuator may bias the control device into an open or closed configuration. In one known assembly, the diaphragm is fixed to the diaphragm plate with an adhesive. In another known assembly, the diaphragm plate includes a dished portion, against which the diaphragm is sealed with a standard worm gear hose-clamp. In other known assemblies, the diaphragm is not fixed to the plate at all. However, the one or more diaphragm plates, as mentioned, are rigidly fixed to the stem of the actuator. Such fixation is generally achieved by threaded attachment. For example, in one form, the stem includes a threaded end portion disposed through a central aperture in the one or more plates. A nut is then threaded onto the threaded end portion of the stem to attach the stem to the plate(s). Additionally, however, a fluid-tight seal must be provided within the housing and between the opposing sides of the diaphragm assembly to enable accurate control of the pneumatic actuator. In one known diaphragm actuator according to that just described, one or more o-rings are provided between the stem and the plate(s).
Each of these known diaphragm assemblies require additional parts such as the o-rings and/or the hose-clamps, and therefore require additional assembly steps. Moreover, these additional parts are prone to failing, thereby decreasing the reliability of the overall device. Furthermore, such known diaphragm actuators are generally configured to operate in a single manner, for example, in either a biased open or a biased closed configuration. Thus, if and when a change in the biased configuration of the actuator is desired, a completely different actuator must be installed.